Ernests Gulbis overpowered second-seeded Juan Martin del Potro to reach the semi-finals of the Farmers\' Classic Friday, booking his first semi-final since January. Gulbis, ranked 84th in the world, pounced early and never let up in a 6-2, 6-4 victory over the 2009 US Open champion. He broke Argentina\'s del Potro in the first game and again in the fifth as he took the opening set with ease. Del Potro battled to gain a 3-1 lead with a break in the second set, but his frustration was clear when he dropped his serve in the next game. Gulbis broke again in the ninth to lead 5-4. He needed four match points to close it out, finally wrapping it up with a forehand winner \"It was very difficult,\" said del Potro, who never looked comfortable on court. \"I got frustrated in the second set when I broke his serve but he broke me back. He was playing with a lot of confidence and took his chances. \"When you play like I did against a player like him, you lose.\" The setback was especially disappointing as del Potro looks ahead to the 2011 US Open. He was unable to defend his title at Flushing Meadows after a wrist injury cut short his 2010 season. His injury layoff saw his ranking slide to outside the top 400 and he has painstakingly worked it back up to 19. \"I just need to keep working on my game, I don\'t know any other way,\" del Potro said. \"But I\'m still happy with my level. \"I\'m enjoying playing this season, I did well on clay and the grass.\" Gulbis, owner of one career title, is trying to get his season back on track. The Latvian rose as high as 21 in the world in February, but has fallen to 84th after a string of lackluster results. \"This is probably one of the most important wins of my career,\" he said. Gulbis next faces his doubles partner, American Alex Bogomolov, who pulled off another upset with a 1-6, 6-4, 6-1 victory over fourth-seeded Brazilian Thomaz Bellucci. The other semi-final has a familiar ring as teenager Ryan Harrison will get a semi-final shot at top-seeded Mardy Fish for the second straight week. Harrison, the 19-year-old who reached his first career semi-final in Atlanta last week, showed that was no fluke as he rallied to beat Taiwan\'s Lu Yen-Hsun 3-6, 6-3, 6-4. Fish advanced with a 6-2, 6-4 victory over eighth-seeded Russian Igor Kunitsyn. \"It was a little awkward,\" world number nine Fish said of a match that started with five straight breaks of serve. \"Luckily I was able to maintain the advantage, get a hold and get in front.\" Fish went up a break in the fifth game of the second set and maintained the advantage the rest of the way. Harrison played the first ATP semi-final of his career last week in Atlanta, falling to world number nine Fish, who went on to capture his second straight Atlanta crown. \"Obviously Mardy\'s been playing fantastic,\" Harrison said. \"He\'s in the top 10 in the world. I had a tough one against him last week. I\'d love to have another shot at him.\" Lu, 27, had beaten third-seeded Cypriot Marcos Baghdatis to reach the quarter-finals here and gained the advantage against Harrison when he broke for a 5-3 lead then served out the first set. After an early exchange of breaks in the second set, Lu wasted a bevy of break chances in the seventh game as Harrison held serve. The Taiwanese player never really recovered. After that, Lu said, \"I started to sink a little bit.\" \"I had a lot of chances,\" Lu said. \"But I didn\'t play well enough on the key points.\"